Instrumentation and Controls Division Electronic Systems Section

Instrumentation and Controls Division Electronic Systems Section Micro-Miniature Radio Frequency Transmitter for Communication and Tracking Applicati...
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Instrumentation and Controls Division Electronic Systems Section

Micro-Miniature Radio Frequency Transmitter for Communication and Tracking Applications

R. I. Crutcher M. S. Emery K. G. Falter C. H. Nowlin Oak Ridge National Laboratory* P. 0. Box 2008 Oak Ridge, Tennessee 3783 1-6006

423-574-5630

J. M. Rochelle L. G. Clonts

University of Tennessee Electrical Engineering Department Knoxville, Tennessee 37996-2100 Paper for submission to: First Annual Symposium on Enabling Technologies for Law Enforcement and Security Boston, Massachusetts November 19-21, 1996 ‘The submitted manuscript has been authored by a con!sactor to the US. Government under contract No. DE-ACU5-96OR22464. Accordingly. the US.Government ntains a nonexciusive. royalty-Em l i a n a to publish or repmduce Le published form of this contribution, or allow others to do so, for US.Government purposes.”

*Managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy under contract DE-AC05-960R22464.

DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employets, makes any warranty, exprw or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its usc would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise dots not necessarily constitute or impiy its endorsement, recommendirtion, or favoring by the United States Government or any agency thereof. The views and opinions of authors ~ X P T ~ S S C . herein ~ do not neassarily state or reflect those of the United States Government or any agency thereof.

Microminiature radio-frequency transmitter for communication and tracking applications

R. I. Crutcher, M. S. Emery, K. G. Falter, andC. H. Nowlin Oak Ridge National Laboratory Oak Ridge, Tennessee 37831-6006

J. M. Rochelle and L. G. Clonts University of Tennessee Electrical Engineering Department Knoxville, Tennessee 37996-2100

ABSTRACT A micro-miniature radio frequency (rf) transmitter has been developed and demonstrated by the Oak Ridge National Laboratory. The objective of the rf transmitter development was to maximize the transmission distance while drastically shrinking the overall transmitter size, including antenna. Based on analysis and testing, an application-specific integrated circuit (ASIC) with a 16-GHz gallium arsenide (GaAs) oscillator and integrated onchip antenna was designed and fabricated using microwave monolithic integrated circuit (MMIC) technology. Details of the development and the results of various field tests will be discussed. The rf transmitter is applicable to covert surveillance and tracking scenarios due to its small size of 2.2 x 2.2 mm, including the antenna. Additionally, the 16-GHz frequency is well above the operational range of consumer-grade radio scanners, providing a degree of protection from unauthorized interception. Variations of the transmitter design have been demonstrated for tracking and tagging beacons, transmission of digital data, and transmission of real-time analog video from a surveillance camera. Preliminary laboratory measurements indicate adaptability to direct-sequence spread-spectrum transmission, providing a low probability of intercept and/or detection. Concepts related to law enforcement applications will be presented.

Keywords: beacon, transmitter, microwave, MMIC, ASIC, GaAs

1. INTRODUCTION The Instrumentation and Controls Division of Oak Ridge National Laboratory (ORNL) has developed a microminiature radio frequency Q transmitter. The device is an rf successor to a pulsed-mode miniature infrared (IR) transmitter previously developed for field studies of Africanized bees.' The focus of the rf transmitter development was to maximize the transmission distance while drastically shrinking the overall transmitter size, including an on-board antenna. A theoretical analysis was conducted to identify an antenna geometry that could be miniaturized and fabricated using standard integrated circuit technology. Based on this analysis, an applicationspecific integrated circuit (ASIC) with a 16-GHz gallium arsenide (GaAs) oscillator and integrated on-chip antenna was designed, fabricated, and tested. This paper addresses the design of the transmitter and describes implementations directly applicable to law enforcement applications.

2. ANTENNA DEVELOPMENT A major design goal of the transmitter development was to make the entire system as small as possible. Because no size advantage would be gained from designing a miniature transmitter that attached to a large external antenna, the first design goal was to determine if the antenna could be fabricated onto the integrated circuit (IC) chip. A theoretical analysis with extensive modeling was performed with the objective of maximizing the transmission distance while fitting the antenna onto the same chip as the active electronics.

Several assumptions were made concerning the transmitting frequency, maximum transmitting antenna size, available rf drive to the transmitting antenna, and the receiving antenna. The transmission frequency was limited to less than 20 GHz, primarily because usable transistor gains at higher frequencies were not obtainable from readily available semiconductor processes. Another frequency constraint was to avoid the microwave-absorption bands in the atmosphere. The size of the transmitting antenna was constrained to a maximum of 2.2 x 2.2 mm, based on the dimensions of a standard semiconductor chip. The antenna drive stage was limited to a performance envelope of either 5 V or 50 mA, whichever was reached first. Additionally, a 60-cm parabolic dish receiving antenna was chosen because one was readily available in our organization. Various antenna configurations were modeled to determine the radiation resistance when implemented as an electrically short antenna. Two configurations that showed promise were the loop antenna and the capacitivelyloaded dipole, also called an “inverted-L”. Simulations indicated that the loop antenna would be the best choice for frequencies above 25 GHz when operated within the other constraints. Because this frequency exceeded the 20-GHz semiconductor process limitation, the inverted-L was selected for the design. Simulations of the invertedL antenna indicated that a performance peak occurred in the frequencyaregion around 16 GHz, which was within the original constraints. Additionally, the calculations indicated that a transmission range of approximately 3 km could be obtained when using the 60-cm receiving antenna. Several prototype antennas were fabricated onto GaAs substrates in both the loop and inverted-L configurations to experimentaIly verify the analytical results. Field strength measurements were made for the two antenna configurations using a microwave generator as a transmitting source and a spectrum analyzer as a calibrated receiver. Polar-coordinate plots were made for the field strength of the two antenna configurations. Test results indicated that the inverted-L provided performance consistent with the theoretical predictions. The next step was to design a transmitter to operate with the antenna.

3. RF CHIP DEVELOPMENT Results of the antenna development phase indicated that an operating frequency in the 16-GHz range was optimum for the inverted-L antenna. Operation at this frequency necessitated the use of a GaAs semiconductor fabrication process for the electronics. An additional benefit of GaAs technology is that the conductivity of GaAs is much lower than for a silicon substrate, thus reducing the rf losses into the substrate when the antenna is fabricated directly on the chip. The “HA” process from TriQuint Semiconductor was chosen to implement the transmitter with on-chip antenna. The HA process supports transistor gate lengths as small as 0.5 micron, which produces transistors with a current gain-bandwidth product of 21 GHz. The process offers two layers of metallization for signal routing. The first layer is deposited on the surface of the GaAs substrate. The second layer is constructed from support post structures under an elevated metal layer that forms an “air-bridge’’ for crossing over other traces. The inverted-L antenna was constructed in the air-bridge layer because it slightly elevated the antenna structure above the substrate and further reduced the substrate losses. The oscillator circuit was implemented as a symmetrically balanced, push-pull, negative-resistance topology. The transistor gate input impedance resonates with on-chip gate inductors to produce a real part of the gate input impedance that is sufficiently negative to sustain oscillation. On-chip inductors and capacitors are implemented as a resonant tuned circuit in the transistor drain path to determine the frequency of oscillation. Additionally, a voltage-dependent capacitance is implemented in the form of a reverse-biased gatekhannel metal semiconductor field effect transistor (MESFET) operating as a varactor diode. This voltage-dependent capacitance makes the frequency of the oscillator adjustable based on a control voltage, thus producing a voltage controlled-oscillator (VCO). The rf output from the oscillator is coupled to the antenna through on-chip inductors that match the antenna impedance to the drive electronics. Additionally, a gatekhannel MESFET diode is included on the chip to provide a method of measuring the temperature of the circuitry for compensation of temperature effects on the oscillator frequency.

The completed 2.2 x 2.2 mm GaAs microtransmitter chip is shown in the microphotograph of Fig. 1. The symmetrical inverted-L antenna can be seen across the top and two sides of the chip. The transmitter electronics are positioned in the central portion of the chip, and electrical connections for power, VCO input, and chip temperature measurement are brought out on the bonding pads shown at the bottom of the photograph.

4. ANTENNA PATTERNS Far-field antenna pattern measurements were conducted to characterize the rf radiation performance of the electrically short antenna fabricated on the GaAs chip. Additionally, the microtransmitter with on-chip antenna was characterized for several mounting and packaging configurations. Because both dielectric and metallic materials in the near-field region of the antenna affect the radiation pattern, the results differ from the pattern of an ideal dipole antenna. The measurement system induded a spectrum analyzer for the test receiver, a 60-cm parabolic receive antenna, and a tripod with a rotational platform to hold the transmitter substrate in the appropriate orientations. The antenna patterns were sampled in 15" increments for each rotational plane. Electromagnetic fields were measured for both horizontal and vertical polarizations for each of the transmitter orientations. Representative samples of the resulting characterizations for the horizontally-polarized case are shown in the polar plots of Fig. 2. These antenna pattern measurements are presented as polar-coordinate plots with the amplitude scale normalized to 0 dB. The plots illustrate the directional properties of the transmitter antenna coverage and provide a peak-tonull performance measurement. The peak-to-null ratio of the directional pattern will scale appropriately for other receiving systems, with the absolute range increasing as the receiver performance improves. Figure 2 (a) shows the antenna pattern as measured in the plane of the transmitter chip. The rf chip characterized in this plot is mounted on a cerainic substrate with additional components that affect the field pattern. The best Performance is on the side of the substrate with the rf chip antenna outward (180°), which is consistent with theoretical dipole performance. A 14-dB null is introduced at 330" by a metallic electronic component mounted on the ceramic substrate. Additional minor signal cancellations at other angles result from other electronics components on the substrate.

Fig. 1. Photomicrograph of GaAs transmitter chip.

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